The present invention relates to a drive power controller for a hybrid vehicle having a motor generator added to a drive system with an engine.
The present application claims priority from Japanese Patent Application No. 2003-388155, the disclosure of which is incorporated herein by reference.
Recently, in a background of environmental and energy-saving issues, attentions are being given to the hybrid vehicle having the motor generator added to the drive system with the engine. The hybrid vehicle of this type selectively couples the motor generator to the engine as a power source for the drive system, thereby providing various modes such as a running mode of using either or both of the engine and the motor generator, a power source braking mode of using either or both of the engine and the motor generator, and an energy regenerative mode of using the motor generator during braking operations. For the hybrid vehicle of this type, developments are being made in an all-wheel drive (AWD) vehicle, which transfers a drive power to both front and rear wheels, in addition to a two-wheel drive vehicle which uses only the front or rear wheels as its drive wheels. Such an AWD vehicle has been proposed which is equipped with a drive power controller for providing a control to a distribution of a torque to the front and rear wheels or the right or left of the front or rear wheels as required.
In Japanese Patent Application Laid-Open No. 2000-43696, an example of such an AWD vehicle is described. The vehicle is not a central differential geared 4WD in which an output torque is transferred from the engine to a central differential via a transmission gear and then to the front and rear wheel drive systems at a distribution ratio defined by the central differential. That is, the motor generator which controllably and variably adds a drive torque or a regenerative braking torque is coupled between the engine and the transmission gear, and the central differential is provided with a differential limiting clutch. The configuration allows the engagement of the differential limiting clutch provided at the central differential to be adjusted, thereby controlling the torque distribution ratio between the front and rear wheels. In accordance with the torque distribution ratio, the total drive torque or the sum of the engine torque and the drive or braking torque from the motor generator is divided to the front and rear wheels.
That is, under the condition in which the clutch for limiting the operation of the central differential is released, the arrangement allows the torque to be distributed to the front and rear wheels at a specific torque distribution ratio (e.g., front wheels:rear wheels=3:7) defined by the gear ratio of the central differential. An adjustment of the clutch engagement allows for variably controlling the torque distribution ratio between the front and rear wheels from the specific torque distribution ratio up to a torque distribution ratio (e.g., front wheels:rear wheels=5:5) at which the clutch is in a direct coupling condition.
According to the aforementioned prior art, it is possible to continuously vary the torque distribution ratio between the front and rear wheels, e.g., from the ratio where the rear wheels has too much of the torque, such as 3:7, to the ratio of 5:5 for the front and rear wheels being in a direct coupling condition. However, it is not possible to continuously vary the torque distribution ratio from the ratio of 5:5 to more than that. However, in order to obtain the optimal torque distribution ratio under various running conditions, for example, the torque distribution which is more than 5:5 to the front wheels ensures a stable running during running straight at a constant speed or on a downhill. If the torque distribution to the rear wheels is made larger, it ensures start and acceleration performances upon an acceleration such as a starting of the vehicle. It is thus required that the torque distribution ratio can be changed in a wide range, for example, from the ratio of less than 5:5 to the ratio of more than 5:5.
Now, suppose that a difference in a rotation occurs between the front and rear wheel output components of the central differential upon cornering or running on a road with a low friction coefficient μ. In this case, the aforementioned prior art allows the clutch to produce a differential limiting torque as required so as to increase the low rotational speed side transfer torque as the differential resisting torque, thereby varying the torque distribution ratio between the front and rear wheels. That is, according to the prior art, the torque cannot be moved between the front and rear wheels in the absence of the difference in a rotational speed between the front and rear wheel output components of the central differential, while the higher rotational speed side torque cannot be increased with the higher rotational speed remaining unchanged. Accordingly, when an active change in the torque distribution ratio is positively attempted between the front and rear wheels on an assumption of various situations to improve running performance, there is a problem in which a limitation to the attempt may occur.
Furthermore, in the aforementioned prior art, the differential limiting clutch restricts differential rotations to make the change in the torque distribution ratio, thereby naturally causing a degradation in a differential function with the change in the torque distribution ratio. That is, the prior art imposes a limitation that the torque distribution ratio cannot be actively changed in a case where the differential function is required. A problem that the limitation compromises the active change in the torque distribution ratio to be made according to various situations may be thus raised, thereby reducing the number of torque distribution control variations.
Now, an attention is focused on the relation between the functionality of the hybrid vehicle coupled with the motor generator and the torque distribution ratio control. In the aforementioned prior art, since the differential limiting clutch of the central differential provides a control to the torque distribution ratio, the control is performed separately from the drive or regenerative braking control provided by the motor generator. Accordingly, when the change in the torque distribution ratio is made between the front and rear wheels in connection with the drive and regenerative braking control by the motor generator, the control must be provided for two operations, i.e., the respective controls of the differential limiting clutch and the motor generator, thereby making the control complicated. The prior art cannot be said to be sufficient in terms of an efficient energy utilization because a braking torque produced to change the torque distribution ratio between the front and rear wheels cannot be used for the energy regeneration in the motor generator.